From July 16, 2018 Google Maps is no longer free, it means that you have to associate a credit card so that you can get billed if your site has requests that exceed the $200 credit they give you monthly for free. For more information, you could consult the price here: https://cloud.google.com/maps-platform/pricing/

In order to display a Google Map on your listings you need to get a Google Maps API key registered to your website (Register for a Google Maps API Key here) then enter the key into the Google Maps setting in Real Estate Options.